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15-441 Computer Networking. Network Security: Cryptography, Authentication, Integrity. Foundations: what is security? cryptography authentication message integrity key distribution and certification Security in practice: application layer: secure e-mail

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15-441 Computer Networking

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15-441 Computer Networking

Network Security:

Cryptography, Authentication, Integrity


what is security?



message integrity

key distribution and certification

Security in practice:

application layer: secure e-mail

transport layer: Internet commerce, SSL, SET

network layer: IP security

Chapter 7: Network security

Lecture #18: 11-06-01

Friends and enemies: Alice, Bob, Trudy

  • well-known in network security world

  • Bob, Alice (lovers!) want to communicate “securely”

  • Trudy, the “intruder” may intercept, delete, add messages

Figure 7.1 goes here

Lecture #18: 11-06-01

What is network security?

Secrecy: only sender, intended receiver should “understand” msg contents

  • sender encrypts msg

  • receiver decrypts msg

    Authentication: sender, receiver want to confirm identity of each other

    Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection

Lecture #18: 11-06-01





The language of cryptography



symmetric key crypto: sender, receiver keys identical

public-key crypto: encrypt key public, decrypt key secret


Figure 7.3 goes here

Lecture #18: 11-06-01

Symmetric key cryptography

substitution cipher: substituting one thing for another

  • monoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq


Plaintext: bob. i love you. alice

ciphertext: nkn. s gktc wky. mgsbc

  • Q: How hard to break this simple cipher?:

    • brute force (how hard?)

    • other?

Lecture #18: 11-06-01

Symmetric key crypto: DES

DES: Data Encryption Standard

  • US encryption standard [NIST 1993]

  • 56-bit symmetric key, 64 bit plaintext input

  • How secure is DES?

    • DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months

    • no known “backdoor” decryption approach

  • making DES more secure

    • use three keys sequentially (3-DES) on each datum

    • use cipher-block chaining

Lecture #18: 11-06-01

DES operation

Symmetric key crypto: DES

initial permutation

16 identical “rounds” of function application, each using different 48 bits of key

final permutation

Lecture #18: 11-06-01

Public Key Cryptography

symmetric key crypto

  • requires sender, receiver know shared secret key

  • Q: how to agree on key in first place (particularly if never “met”)?

public key cryptography

  • radically different approach [Diffie-Hellman76, RSA78]

  • sender, receiver do not share secret key

  • encryption key public (known to all)

  • decryption key private (known only to receiver)

Lecture #18: 11-06-01

Public key cryptography

Figure 7.7 goes here

Lecture #18: 11-06-01

d (e (m)) = m





need public and private keys

for d ( ) and e ( )





Public key encryption algorithms

need d ( ) and e ( ) such that

Two inter-related requirements:





RSA: Rivest, Shamir, Adleman algorithm

Lecture #18: 11-06-01

RSA: Choosing keys

1. Choose two large prime numbers p, q.

(e.g., 1024 bits each)

2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors

with z. (e, z are “relatively prime”).

4. Choose d such that ed-1 is exactly divisible by z.

(in other words: ed mod z = 1 ).

5.Public key is (n,e).Private key is (n,d).

Lecture #18: 11-06-01

1. To encrypt bit pattern, m, compute



c = m mod n

m = c mod n


(i.e., remainder when m is divided by n)





m = (m mod n)

mod n

RSA: Encryption, decryption

0. Given (n,e) and (n,d) as computed above

2. To decrypt received bit pattern, c, compute


(i.e., remainder when c is divided by n)

Lecture #18: 11-06-01



m = c mod n

c = m mod n



RSA example:

Bob chooses p=5, q=7. Then n=35, z=24.

e=5 (so e, z relatively prime).

d=29 (so ed-1 exactly divisible by z.

















Lecture #18: 11-06-01




(m mod n)

mod n = m mod n

ed mod (p-1)(q-1)


= m mod n

= m

= m mod n

Number theory result: If p,q prime, n = pq, then


y mod (p-1)(q-1)



x mod n = x mod n

m = (m mod n)

mod n

RSA: Why:


Notice that

(mdmod n) emod n

= mde mod n

= m

eB(dB(m)) = m

(using number theory result above)

(since we choseed to be divisible by

(p-1)(q-1) with remainder 1 )

Lecture #18: 11-06-01


Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0:Alice says “I am Alice”

Failure scenario??

Lecture #18: 11-06-01

Authentication: another try

Protocol ap2.0:Alice says “I am Alice” and sends her IP

address along to “prove” it.

Failure scenario??

Lecture #18: 11-06-01

Authentication: another try

Protocol ap3.0:Alice says “I am Alice” and sends her

secret password to “prove” it.

Failure scenario?

Lecture #18: 11-06-01

Authentication: yet another try

Protocol ap3.1:Alice says “I am Alice” and sends her

encrypted secret password to “prove” it.

I am Alice


Failure scenario?

Lecture #18: 11-06-01

Authentication: yet another try

Goal:avoid playback attack

Nonce:number (R) used onlyonce in a lifetime

ap4.0:to prove Alice “live”, Bob sends Alice nonce, R. Alice

must return R, encrypted with shared secret key

Figure 7.11 goes here

Failures, drawbacks?

Lecture #18: 11-06-01

Authentication: ap5.0

ap4.0 requires shared symmetric key

  • problem: how do Bob, Alice agree on key

  • can we authenticate using public key techniques?

    ap5.0: use nonce, public key cryptography

Figure 7.12 goes here

Lecture #18: 11-06-01

ap5.0: security hole

Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)

Figure 7.14 goes here

Need “certified” public

keys (more later …)

Lecture #18: 11-06-01

Cryptographic technique analogous to hand-written signatures.

Sender (Bob) digitally signs document, establishing he is document owner/creator.

Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document.

Simple digital signature for message m:

Bob encrypts m with his public key dB, creating signed message, dB(m).

Bob sends m and dB(m) to Alice.

Digital Signatures

Lecture #18: 11-06-01

Suppose Alice receives msg m, and digital signature dB(m)

Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.

If eB(dB(m) ) = m, whoever signed m must have used Bob’s private key.

Alice thus verifies that:

Bob signed m.

No one else signed m.

Bob signed m and not m’.


Alice can take m, and signature dB(m) to court and prove that Bob signed m.

Digital Signatures (more)

Lecture #18: 11-06-01

Computationally expensive to public-key-encrypt long messages

Goal: fixed-length,easy to compute digital signature, “fingerprint”

apply hash function H to m, get fixed size message digest, H(m).

Hash function properties:


Produces fixed-size msg digest (fingerprint)

Given message digest x, computationally infeasible to find m such that x = H(m)

computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).

Message Digests

Lecture #18: 11-06-01

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

Digital signature = Signed message digest

Lecture #18: 11-06-01

Internet checksum would make a poor message digest.

Too easy to find two messages with same checksum.

MD5 hash function widely used.

Computes 128-bit message digest in 4-step process.

arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.

SHA-1 is also used.

US standard

160-bit message digest

Hash Function Algorithms

Lecture #18: 11-06-01


How do two entities establish shared secret key over network?


trusted key distribution center (KDC) acting as intermediary between entities


When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?


trusted certification authority (CA)

Trusted Intermediaries

Lecture #18: 11-06-01

Alice,Bob need shared symmetric key.

KDC: server shares different secret key with each registered user.

Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC.

Key Distribution Center (KDC)

  • Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1)

  • Alice sends Bob KB-KDC(A,R1), Bob extracts R1

  • Alice, Bob now share the symmetric key R1.

Lecture #18: 11-06-01

Certification authority (CA) binds public key to particular entity.

Entity (person, router, etc.) can register its public key with CA.

Entity provides “proof of identity” to CA.

CA creates certificate binding entity to public key.

Certificate digitally signed by CA.

When Alice wants Bob’s public key:

gets Bob’s certificate (Bob or elsewhere).

Apply CA’s public key to Bob’s certificate, get Bob’s public key

Certification Authorities

Lecture #18: 11-06-01

Secure e-mail

  • Alice wants to send secret e-mail message, m, to Bob.

  • generates random symmetric private key, KS.

  • encrypts message with KS

  • also encrypts KS with Bob’s public key.

  • sends both KS(m) and eB(KS) to Bob.

Lecture #18: 11-06-01

Secure e-mail (continued)

  • Alice wants to provide sender authentication message integrity.

  • Alice digitally signs message.

  • sends both message (in the clear) and digital signature.

Lecture #18: 11-06-01

Secure e-mail (continued)

  • Alice wants to provide secrecy, sender authentication, message integrity.

Note: Alice uses both her private key, Bob’s public key.

Lecture #18: 11-06-01

Internet e-mail encryption scheme, a de-facto standard.

Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

Provides secrecy, sender authentication, integrity.

Inventor, Phil Zimmerman, was target of 3-year federal investigation.


Hash: SHA1

Bob:My husband is out of town tonight.Passionately yours, Alice


Version: PGP 5.0

Charset: noconv



Pretty good privacy (PGP)

A PGP signed message:

Lecture #18: 11-06-01

PGP provides security for a specific network app.

SSL works at transport layer. Provides security to any TCP-based app using SSL services.

SSL: used between WWW browsers, servers for I-commerce (shttp).

SSL security services:

server authentication

data encryption

client authentication (optional)

Server authentication:

SSL-enabled browser includes public keys for trusted CAs.

Browser requests server certificate, issued by trusted CA.

Browser uses CA’s public key to extract server’s public key from certificate.

Visit your browser’s security menu to see its trusted CAs.

Secure sockets layer (SSL)

Lecture #18: 11-06-01

Encrypted SSL session:

Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

Using its private key, server decrypts session key.

Browser, server agree that future msgs will be encrypted.

All data sent into TCP socket (by client or server) is encrypted with session key.

SSL: basis of IETF Transport Layer Security (TLS).

SSL can be used for non-Web applications, e.g., IMAP.

Client authentication can be done with client certificates.

SSL (continued)

Lecture #18: 11-06-01

designed for payment-card transactions over Internet.

provides security services among 3 players:



merchant’s bank

All must have certificates.

SET specifies legal meanings of certificates.

apportionment of liabilities for transactions

Customer’s card number passed to merchant’s bank without merchant ever seeing number in plain text.

Prevents merchants from stealing, leaking payment card numbers.

Three software components:

Browser wallet

Merchant server

Acquirer gateway

See text for description of SET transaction.

Secure electronic transactions (SET)

Lecture #18: 11-06-01

Network-layer secrecy:

sending host encrypts the data in IP datagram

TCP and UDP segments; ICMP and SNMP messages.

Network-layer authentication

destination host can authenticate source IP address

Two principle protocols:

authentication header (AH) protocol

encapsulation security payload (ESP) protocol

For both AH and ESP, source, destination handshake:

create network-layer logical channel called a service agreement (SA)

Each SA unidirectional.

Uniquely determined by:

security protocol (AH or ESP)

source IP address

32-bit connection ID

Ipsec: Network Layer Security

Lecture #18: 11-06-01

Provides secrecy, host authentication, data integrity.

Data, ESP trailer encrypted.

Next header field is in ESP trailer.

ESP authentication field is similar to AH authentication field.

Protocol = 50.

ESP Protocol

Lecture #18: 11-06-01

Provides source host authentication, data integrity, but not secrecy.

AH header inserted between IP header and IP data field.

Protocol field = 51.

Intermediate routers process datagrams as usual.

AH header includes:

connection identifier

authentication data: signed message digest, calculated over original IP datagram, providing source authentication, data integrity.

Next header field: specifies type of data (TCP, UDP, ICMP, etc.)

Authentication Header (AH) Protocol

Lecture #18: 11-06-01

Network Security (summary)

Basic techniques…...

  • cryptography (symmetric and public)

  • authentication

  • message integrity

    …. used in many different security scenarios

  • secure email

  • secure transport (SSL)

  • IP sec

See also: firewalls , in network management

Lecture #18: 11-06-01

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